This invention relates to an apparatus for sampling soil gases, for example, an anomolous concentration of helium or other gases, which may be indicative of nearby hydrocarbon deposits, a geothermal reservoir, or deposits of radioactive ores.
Most rapid surveys which collect subsurface gaseous emanations use a probe that collects samples at a depth from about 18" to about 36" below ground surface. A shaft is driven to the desired depth, and a gas sample is taken through an aperture near the lower end of the shaft. At this depth the gas sample is a mixture of subsurface gases rising from the resource, biogenically produced gases, and air penetrating down from the surface. Dilution of the subsurface gas with air from the surface is undesirable, as it both dilutes gas concentrations and introduces foreign gases into the sample. Contamination of the sample by surface air can be minimized by driving a shaft into the earth, the dimensions of the shaft being such that it fills the cavity made as it is driven into the earth. A gas seal formed by the exterior wall of the shaft and the surrounding earth mitigates contamination of the sample by atmospheric air. Dilution of the sample by surface air can be further minimized by collecting the samples of gas at a depth of greater than about 6 feet.
Obstacles, such as packed gravel and large rocks, are frequently encountered as the shaft is sunk into the earth. As the probability of encountering such obstacles increases with the increasing depth to which the shaft is sunk, an apparatus for taking subsurface gas samples at for example a depth of 10 feet must be durable, easily repaired, and have good penetrating abilities.
Soil gas helium concentration is commonly only a few parts per million. Because helium is soluble in water, a helium sample taken from a high moisture content site can yield misleading results. It is thus important a gas probe have the capability of obtaining a soil sample and reliably transporting the same to surface for a moisture analysis.